function onMenu(num){
    for(let i=0;i<4;i++) {
        document.getElementById("menu_" + i).style.backgroundColor=num===i?"#fff4":"#0000";
        if(i===num)document.getElementById("body").src=url_header+"/html/page_"+i+url_ender;
    }
    window.sessionStorage.setItem("select_page",num);
}
let select_page=window.sessionStorage.getItem("select_page");
if(select_page===null || select_page===undefined)onMenu(0);
else onMenu(parseInt(select_page));